1. Field of the Invention
The present invention relates to a method and apparatus for microencapsulation of electroluminescent phosphor particles to provide protection against moisture and to improve product life expectancy.
2. Description of the Prior Art
A good review of electroluminescent devices appears in the paper entitled "Electroluminescent Lighting Applications" by Chesley S. Pieroway, which was presented to the Illuminating Engineering Society of North America, Aviation Lighting Committee, in November 1981. As explained in that paper, there are presently two basic types of electroluminescent phosphor lighting, referred to as the thin film and the thick film types. The thin film type uses a vacuum deposition procedure at about 500.degree. to 600.degree. F. (260.degree. to 318.degree. C.) to make a very thin coating on glass. This system requires a high voltage and high frequency energization, typically 900 volts and 20,000 Hertz but possesses extreme brightness. The thin film system, however, is very sensitive to power changes. It is expensive, heavy, and is available only in a limited number of colors.
The thick film device provides a phosphor thickness measured in mils and can be applied by means of a silk screen or a doctor blade on an aluminum foil. It operates on standard alternating current voltage at frequencies of 60 to 20,000 Hertz. The lamps are available in many colors at a reasonable output and are relatively inexpensive.
In recent times, there have been suggestions made for encapsulating the electroluminescent phosphors such as zinc sulfide in an attempt to prevent attack by moisture. Probably the most successful of these has been microencapsulation with urethane type polymers. These polymers are applied by a coacervation process wherein the particles to be coated are suspended in a toluene solution and cooled. A hydrolyzed ethylene acetate is deposited on the particle walls and then the coating is reacted with a cross-linking agent such as a diisocyanate to form a urethane type material. At best, however, the minimum thickness of the walls using this type of procedure is limited to about 0.1 micron. Similar techniques have been used to produce microencapsulated materials from polymers such as cyano-vinyl polymers, cross-linked cyano-ethyl cellulose polymers, and urea-formaldehyde.